A088836 a(n) = (A020639(2n+1) + A006530(2n+1))/2, arithmetic mean of max and min prime factor applied to odd numbers (i.e., when this mean is an integer).

3, 5, 7, 3, 11, 13, 4, 17, 19, 5, 23, 5, 3, 29, 31, 7, 6, 37, 8, 41, 43, 4, 47, 7, 10, 53, 8, 11, 59, 61, 5, 9, 67, 13, 71, 73, 4, 9, 79, 3, 83, 11, 16, 89, 10, 17, 12, 97, 7, 101, 103, 5, 107, 109, 20, 113, 14, 8, 12, 11, 22, 5, 127, 23, 131, 13, 4, 137, 139, 25, 12, 17, 5, 149, 151

Offset

1,1

Links

Michael De Vlieger, Table of n, a(n) for n = 1..10000

Example

If 2n+1 is prime then a(2n+1) = 2n + 1.

Mathematica

Table[(FactorInteger[2 n + 1][[1, 1]] + FactorInteger[2 n + 1][[-1, 1]])/2, {n, 75}] (* Michael De Vlieger, Mar 28 2015 *)

Prog

(PARI) a(n) = {my(p = factor(2*n+1)[, 1]); (p[1] + p[#p])/2; } \\ Amiram Eldar, Mar 18 2025

Crossrefs

Cf. A020639, A006530.

Sequence in context: A326577 A090368 A120374 * A076565 A286517 A377865

Adjacent sequences: A088833 A088834 A088835 * A088837 A088838 A088839

Keyword

nonn

Author

Labos Elemer, Oct 31 2003

Status

approved